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A simple light water reactor

Light water reactorFrom Wikipedia, the free encyclopedia

The light water reactor (LWR) is a type of thermal reactor that uses normalwater, as opposed to heavy water, as its coolant and neutron moderator and asolid compound of fissile element as its fuel. Thermal reactors are the mostcommon type of nuclear reactor, and light water reactors are the most commontype of thermal reactor. There are three varieties of light water reactors: thepressurized water reactor (PWR), the boiling water reactor (BWR), and (mostdesigns of) the supercritical water reactor (SCWR).

Contents

1 History

1.1 Early concepts and experiments1.2 Naval reactor development and first Pressurized WaterReactor

1.3 Borax Experiments and first Boiling Water Reactor2 Overview

2.1 LWR Statistics3 Reactor design

3.1 Control

3.2 Coolant3.3 Fuel

3.4 Moderator4 PIUS reactor

5 See also6 References

7 External links

History

Early concepts and experiments

After the discoveries of fission, moderation and of the theoretical possibility of a nuclear chain reaction, earlyexperimental results rapidly showed that natural uranium could only undergo a sustained chain reaction using graphite orheavy water as a moderator. While the world's first reactors (CP-1, X10 etc.) were successfully reaching criticality,uranium enrichment began to develop from theoretical concept to practical applications in order to meet the goal of theManhattan Project, to build a nuclear explosive.

In May 1944, the first grams of enriched uranium ever produced reached criticality in the LOPO reactor at Los

Alamos, which was used to estimate the critical mass of U235 to produce the atomic bomb.[1] LOPO cannot beconsidered as the first light water reactor because its fuel was not a solid uranium compound cladded with corrosion-

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The Koeberg nuclear power station, consisting of

two pressurized water reactors fueled with

uranium.

resistant material, but was composed of uranyl sulfate salt dissolved in water.[2] It is however the first aqueous

homogeneous reactor and the first reactor using enriched uranium as fuel and ordinary water as a moderator.[1]

By the end of the war, following an idea of Alvin Weinberg, natural uranium fuel elements were arranged in a lattice in

ordinary water at the top of the X10 reactor to evaluate the neutron multiplication factor.[3] The purpose of thisexperience was to determine the feasibility of a nuclear reactor using light water as a moderator and coolant, and

cladded solid uranium as fuel. The results showed that, with a lightly enriched uranium, criticality could be reached.[4]

This experience was the first practical step toward light water reactor.

After World War II and with the availability of enriched uranium, new concepts of reactor became feasible. In 1946,Eugene Wigner and Alvin Weinberg proposed and developed the concept of a reactor using enriched uranium as a

combustible, and light water as a moderator and coolant.[3] This concept was proposed for a reactor whose purposewas to test the behavior of materials under neutron flux. This reactor, the Material Testing Reactor (MTR), was built in

Idaho at INL and reached criticality on March 31, 1952.[5] For the design of this reactor, experiments were necessary,so a mock-up of the MTR was built at ORNL, to assess the hydraulic performances of the primary circuit and then totest its neutronic characteristics. This MTR mock-up, later called the Low Intensity Test Reactor (LITR), reached

criticality on February 4, 1950[6] and was the world's first light water reactor.[7]

Naval reactor development and first Pressurized Water Reactor

An effort by the United States Navy, starting immediately after the end of World War II, and led by (then) CaptainHyman Rickover, developed the first pressurized water reactors in the early 1950s, building the first nuclear submarine(the USS Nautilus (SSN-571)). The Soviet Union also independently developed their version of the PWR in the late1950s, and it became known as the VVER; because of this, Russian-designed PWRs are known in the West asVVERs, to denote their independent origin, and certain national design distinctions from Western PWRs.

Borax Experiments and first Boiling Water Reactor

Researcher Samuel Untermyer II led the effort to develop the BWR at the US National Reactor Testing Station (nowthe Idaho National Laboratory) in a series of tests called the BORAX experiments.

Overview

The family of nuclear reactors known as light water reactors(LWR), cooled and moderated using ordinary water, tend to besimpler and cheaper to build than other types of nuclearreactor; due to these factors, they make up the vast majority ofcivil nuclear reactors and naval propulsion reactors in servicethroughout the world as of 2009. LWRs can be subdivided intothree categories - pressurized water reactors (PWRs), boilingwater reactors (BWRs), and supercritical water reactors(SWRs). The SWR remains hypothetical as of 2009; it is aGeneration IV design that is still a light water reactor, but it isonly partially moderated by light water and exhibits certaincharacteristics of a fast neutron reactor.

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The leaders in national experience with PWRs, offering reactors for export, are the United States (which offers thepassively-safe AP1000, a Westinghouse design, as well as several smaller, modular, passively-safe PWRs, such as theBabcock and Wilcox MPower, and the NuScale MASLWR), the Russian Federation (offering both the VVER-1000and the VVER-1200 for export), the Republic of France (offering the AREVA EPR for export), and Japan (offeringthe Mitsubishi Advanced Pressurized Water Reactor for export); in addition, both the People's Republic of China andthe Republic of Korea are both noted to be rapidly ascending into the front rank of PWR-constructing nations as well,with the Chinese being engaged in a massive program of nuclear power expansion, and the Koreans currently designingand constructing their second generation of indigenous designs. The leaders in national experience with BWRs, offeringreactors for export, are the United States and Japan, with the alliance of General Electric (of the US) and Hitachi (ofJapan), offering both the Advanced Boiling Water Reactor (ABWR) and the Economic Simplified Boiling WaterReactor (ESBWR) for construction and export; in addition, Toshiba offers an ABWR variant for construction in Japan,as well. West Germany was also once a major player with BWRs. The other types of nuclear reactor in use for powergeneration are the heavy water moderated reactor, built by Canada (CANDU) and the Republic of India (AHWR), theadvanced gas cooled reactor (AGCR), built by the United Kingdom, the liquid metal cooled reactor (LMFBR), builtby the Russian Federation, the Republic of France, and Japan, and the graphite-moderated, water-cooled reactor(RBMK), found exclusively within the Russian Federation and former Soviet states.

Though electricity generation capabilities are comparable between all these types of reactor, due to the aforementionedfeatures, and the extensive experience with operations of the LWR, it is favored in the vast majority of new nuclear

power plants, though the CANDU/AHWR has a comparatively small (but quite dedicated) following.[citation needed] Inaddition, light water reactors make up the vast majority of reactors that power naval nuclear-powered vessels. Four outof the five great powers with nuclear naval propulsion capacity use light water reactors exclusively: the British RoyalNavy, the Chinese People's Liberation Army Navy, the French Marine nationale, and the United States Navy. Only theRussian Federation's Navy has used a relative handful of liquid-metal cooled reactors in production vessels, specificallythe Alfa class submarine, which used lead-bismuth eutectic as a reactor moderator and coolant, but the vast majority ofRussian nuclear-powered boats and ships use light water reactors exclusively. The reason for near exclusive LWR useaboard nuclear naval vessels is the level of inherent safety built into these types of reactors. Since light water is used asboth a coolant and a neutron moderator in these reactors, if one of these reactors suffers damage due to military action,leading to a compromise of the reactor core's integrity, the resulting release of the light water moderator will act to stopthe nuclear reaction and shut the reactor down. This capability is known as a negative void coefficient of reactivity.

Currently-offered LWRs include the following:

ABWRAP1000

ESBWREuropean Pressurized Reactor

VVER

LWR Statistics

Data from the International Atomic Energy Agency.[8]

Reactors in operation. 359

Reactors under construction. 27

Number of countries with LWRs. 27

Generating capacity (Gigawatt). 328.4

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Animated diagram of a pressurized water reactor Animated Diagram of a boiling water reactor

Reactor design

The light water reactor produces heat by controlled nuclear fission. The nuclear reactor core is the portion of a nuclearreactor where the nuclear reactions take place. It mainly consists of nuclear fuel and control elements. The pencil-thinnuclear fuel rods, each about 12 feet (3.7 m) long, are grouped by the hundreds in bundles called fuel assemblies.Inside each fuel rod, pellets of uranium, or more commonly uranium oxide, are stacked end to end. The controlelements, called control rods, are filled with pellets of substances like hafnium or cadmium that readily capture neutrons.When the control rods are lowered into the core, they absorb neutrons, which thus cannot take part in the chainreaction. On the converse, when the control rods are lifted out of the way, more neutrons strike the fissile uranium-235or plutonium-239 nuclei in nearby fuel rods, and the chain reaction intensifies. All of this is enclosed in a water-filledsteel pressure vessel, called the reactor vessel.

In the boiling water reactor, the heat generated by fission turns the water into steam, which directly drives the power-generating turbines. But in the pressurized water reactor, the heat generated by fission is transferred to a secondaryloop via a heat exchanger. Steam is produced in the secondary loop, and the secondary loop drives the power-generating turbines. In either case, after flowing through the turbines, the steam turns back into water in the

condenser.[9]

The water required to cool the condenser is taken from a nearby river or ocean. It is then pumped back into the riveror ocean, in warmed condition. The heat could also be dissipated via a cooling tower into the atmosphere. The UnitedStates uses LWR reactors for electric power production, in comparison to the heavy water reactors used in

Canada.[10]

Control

Main article: Control rod

Control rods are usually combined into control rod assemblies — typically 20 rods for a commercial pressurized waterreactor assembly — and inserted into guide tubes within a fuel element. A control rod is removed from or inserted intothe central core of a nuclear reactor in order to control the number of neutrons which will split further uranium atoms.This in turn affects the thermal power of the reactor, the amount of steam generated, and hence the electricityproduced. The control rods are partially removed from the core to allow a chain reaction to occur. The number ofcontrol rods inserted and the distance by which they are inserted can be varied to control the reactivity of the reactor.

Usually there are also other means of controlling reactivity. In the PWR design a soluble neutron absorber, usually boricacid, is added to the reactor coolant allowing the complete extraction of the control rods during stationary poweroperation ensuring an even power and flux distribution over the entire core. Operators of the BWR design use the

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A pressurized water reactor head, with the

control rods visible on the top.

A nuclear fuel pellet.

coolant flow through the core to control reactivity by varying the speed of the reactor recirculation pumps. An increasein the coolant flow through the core improves the removal of steam bubbles, thus increasing the density of thecoolant/moderator with the result of increasing power.

Coolant

Main article: Nuclear reactor coolant

The light water reactor also uses ordinary water to keep the reactor cooled. The cooling source, light water, iscirculated past the reactor core to absorb the heat that it generates. Theheat is carried away from the reactor and is then used to generatesteam. Most reactor systems employ a cooling system that is physicallyseparate from the water that will be boiled to produce pressurizedsteam for the turbines, like the pressurized water reactor. But in somereactors the water for the steam turbines is boiled directly by the reactorcore, for example the boiling water reactor.

Many other reactors are also light water cooled, notably the RBMKand some military plutonium production reactors. These are notregarded as LWRs, as they are moderated by graphite, and as a resulttheir nuclear characteristics are very different. Although the coolant flowrate in commercial PWRs is constant, it is not in nuclear reactors usedon U.S. Navy ships.

Fuel

Main article: Nuclear fuel

The use of ordinary water makes it necessary to do a certain amount ofenrichment of the uranium fuel before the necessary criticality of thereactor can be maintained. The light water reactor uses uranium 235 asa fuel, enriched to approximately 3 percent. Although this is its majorfuel, the uranium 238 atoms also contribute to the fission process byconverting to plutonium 239; about one-half of which is consumed in thereactor. Light-water reactors are generally refueled every 12 to 18months, at which time, about 25 percent of the fuel is replaced.

The enriched UF6 is converted into uranium dioxide powder that is then

processed into pellet form. The pellets are then fired in a high-temperature, sintering furnace to create hard, ceramic pellets of enricheduranium. The cylindrical pellets then undergo a grinding process to achieve a uniform pellet size. The uranium oxide isdried before inserting into the tubes to try to eliminate moisture in the ceramic fuel that can lead to corrosion andhydrogen embrittlement. The pellets are stacked, according to each nuclear core's design specifications, into tubes ofcorrosion-resistant metal alloy. The tubes are sealed to contain the fuel pellets: these tubes are called fuel rods.

The finished fuel rods are grouped in special fuel assemblies that are then used to build up the nuclear fuel core of apower reactor. The metal used for the tubes depends on the design of the reactor - stainless steel was used in the past,but most reactors now use a zirconium alloy. For the most common types of reactors the tubes are assembled intobundles with the tubes spaced precise distances apart. These bundles are then given a unique identification number,which enables them to be tracked from manufacture through use and into disposal.

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Nuclear fuel pellets that are ready for fuel

assembly completion.

Pressurized water reactor fuel consists of cylindrical rods put intobundles. A uranium oxide ceramic is formed into pellets and insertedinto zirconium alloy tubes that are bundled together. The zirconium alloytubes are about 1 cm in diameter, and the fuel cladding gap is filled withhelium gas to improve the conduction of heat from the fuel to thecladding. There are about 179-264 fuel rods per fuel bundle and about121 to 193 fuel bundles are loaded into a reactor core. Generally, thefuel bundles consist of fuel rods bundled 14x14 to 17x17. PWR fuelbundles are about 4 meters in length. The zirconium alloy tubes arepressurized with helium to try to minimize pellet cladding interactionwhich can lead to fuel rod failure over long periods.

In boiling water reactors, the fuel is similar to PWR fuel except that thebundles are "canned"; that is, there is a thin tube surrounding each bundle. This is primarily done to prevent local densityvariations from effecting neutronics and thermal hydraulics of the nuclear core on a global scale. In modern BWR fuelbundles, there are either 91, 92, or 96 fuel rods per assembly depending on the manufacturer. A range between 368assemblies for the smallest and 800 assemblies for the largest U.S. BWR forms the reactor core. Each BWR fuel rod isback filled with helium to a pressure of about three atmospheres (300 kPa).

Moderator

Main article: Neutron moderator

A neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermalneutrons capable of sustaining a nuclear chain reaction involving uranium-235. A good neutron moderator is a materialfull of atoms with light nuclei which do not easily absorb neutrons. The neutrons strike the nuclei and bounce off. Aftersufficient impacts, the velocity of the neutron will be comparable to the thermal velocities of the nuclei; this neutron isthen called a thermal neutron.

The light water reactor uses ordinary water, also called light water, as its neutron moderator. The light water absorbstoo many neutrons to be used with unenriched natural uranium, and therefore uranium enrichment or nuclearreprocessing becomes necessary to operate such reactors, increasing overall costs. This differentiates it from a heavywater reactor, which uses heavy water as a neutron moderator. While ordinary water has some heavy water moleculesin it, it is not enough to be important in most applications. In practice all LWRs are also water cooled. In pressurizedwater reactors the coolant water is used as a moderator by letting the neutrons undergo multiple collisions with lighthydrogen atoms in the water, losing speed in the process. This moderating of neutrons will happen more often when thewater is denser, because more collisions will occur.

The use of water as a moderator is an important safety feature of PWRs, as any increase in temperature causes thewater to expand and become less dense; thereby reducing the extent to which neutrons are slowed down and hencereducing the reactivity in the reactor. Therefore, if reactivity increases beyond normal, the reduced moderation ofneutrons will cause the chain reaction to slow down, producing less heat. This property, known as the negativetemperature coefficient of reactivity, makes PWR reactors very stable. In event of a loss-of-coolant accident, themoderator is also lost and the active fission reaction will stop leaving just a 5% power level for 1 to 3 years called the"decay heat". This 5% "decay heat" will continue for 1 to 3 years after shut down, whereupon it finally reaches "full coldshutdown". "Decay heat" while dangerous and strong enough to melt the core, is not nearly as dangerous as an activefission reaction. During this "decay heat" post shutdown period the reactor requires water pumped cooling or thereactor will overheat to above 2200 degrees Celsius whereupon the heat separates the cooling water into its constituentparts Hydrogen and Oxygen which can cause hydrogen explosions, threatening structural damage or even the possible

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exposure of highly radioactive stored fuel rods stored ready for use in surrounding nuclear storage pools(approx 15tons of fuel is replenished each year to maintain normal PWR operation). This decay heat is the major risk factor inLWR safety record.

PIUS reactor

PIUS, standing for Process Inherent Ultimate Safety, was a Swedish design concept for a light-water reactor

system.[11] It relied on passive measures, not requiring operator actions or external energy supplies, to provide safeoperation. No units were ever built.

See also

Nuclear powerHeavy water reactorList of nuclear reactorsLight water reactor sustainability

References

1. ̂a b "Federation of American Scientists - Early reactor"(http://www.fas.org/sgp/othergov/doe/lanl/pubs/00416628.pdf). Retrieved 2012-12-30.

2. ^ It also can be noted that as LOPO was designed to operate at zero power, and no means for cooling were necessary,so ordinary water served solely as a moderator.

3. ̂a b "ORNL - An Account of Oak Ridge National Laboratory’s Thirteen Nuclear Reactors"(http://info.ornl.gov/sites/publications/files/Pub20808.pdf). p. 7. Retrieved 2012-12-28. "... Afterwards, responding toWeinberg’s interest, the fuel elements were arranged in lattices in water and the multiplication factors determined. ..."

4. ^ "ORNL - History of the X10 Graphite Reactor" (http://www.ornl.gov/info/news/cco/graphite.shtml). Retrieved 2012-12-30.

5. ^ "INEEL - Proving the principle" (http://www.inl.gov/proving-the-principle/chapter_08.pdf). Retrieved 2012-12-28.

6. ^ "INEL - MTR handbook Appendix F (historical backgroup)"(http://ar.inel.gov/images/pdf/200608/2006080900743TUA.pdf). p. 222. Retrieved 2012-12-31.

7. ^ "DOE oral history presentation program - Interview of LITR operator transcript"(http://www.osti.gov/COROH/ORNL/Transcripts/Beall-Haubenreich%20OH.pdf). p. 4. "... We were so nervousbecause there had never been a reactor fueled with enriched uranium go critical before. ..."

8. ^ "IAEA - LWR" (http://www.iaea.org/NuclearPower/WCR/LWR/). Retrieved 2009-01-18.

9. ^ "European Nuclear Society - Light water reactor"(http://www.euronuclear.org/info/encyclopedia/l/lightwaterreactor.htm). Retrieved 2009-01-18.

10. ^ "Light Water Reactors" (http://hyperphysics.phy-astr.gsu.edu/Hbase/NucEne/ligwat.html). Retrieved 2009-01-18.

11. ^ National Research Council (U.S.). Committee on Future Nuclear Power, Nuclear power: technical and institutionaloptions for the future National Academies Press, 1992, ISBN 0-309-04395-6 page 122

External links

Light Water Reactor Sustainability (LWRS) Program (https://inlportal.inl.gov/portal/server.pt?open=512&objID=442&mode=2&in_hi_userid=2279&cached=true)

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